The present invention relates to an impermeable elastic membrane, to a process for its manufacture and to a hydropneumatic accumulator equipped with this membrane and used, for example, in the suspensions of motor vehicles. Such an accumulator is in the form of a sphere separated by a membrane into two compartments, one of which contains a gas such as nitrogen and the other contains a liquid such as mineral oil. Under the effect of the oil pressure, the membrane becomes deformed and compresses the gas.
The main characteristics of the membrane are its flexibility, elasticity, impermeability to nitrogen, leaktightness and resistance to mineral hydraulic fluid.
Two types of polyurethane-based membrane are currently used, for example a thermoplastic polyurethane such as Desmopan(copyright) from Bayer or a polyurethane combined with rubber such as Urepan(copyright). The durability of these membranes is satisfactory, but they have a drawback as regards the low impermeability to nitrogen of polyurethane. This low impermeability limits the useful life of accumulators of limited. Thus, after three years of use on a vehicle, nearly half of the rating pressure is lost.
It is sought to improve the impermeability to nitrogen of membranes. EP-A-360,648 discloses, in particular, a membrane having twice the impermeability of the polyurethane-based one. This membrane contains two materials in combination, the first material giving the membrane the required elasticity and being selected from thermoplastic polyurethanes, polyether block amides and flexible polyesters or a mixture of these, the second material being immersed in the bulk of the first material in order to achieve the impermeability to gases and selected from an ethylene/vinyl alcohol copolymer, polyamides and polyvinylidene chloride or a mixture of these.
When an ethylene/vinyl alcohol copolymer is used as the second material, it is mixed with a polyether block amide, and the first material is a thermoplastic polyurethane. The thickness of the film is, in this case, typically between 10 and 200 micrometers.
Novel hydropneumatic accumulators have appeared recently on the market. They are described in document FR-A-2,662,638. They contain two compartments separated by a cold-resistant multilayer membrane forming a gas barrier. The film forming the gas barrier is of the polyvinyl alcohol type placed between two sheets of rubber. The role of the rubber is to ensure the leaktightness of the mounting of the membrane in the accumulator. The polyvinyl alcohol film contains a polyol-type plasticizer, for example glycerol, in a percentage of from 15 to 50% by weight.
These novel membranes are satisfactory, but owing to the fact that the plasticizer can migrate or evaporate, especially in the case of use at high temperatures, it is necessary to insert a skin of vapor-impermeable and plasticizer-impermeable resin, for example a polyethylene skin, between the film of polyvinyl alcohol and the rubber. Furthermore, these membranes require specific storage and handling conditions, owing to the presence of plasticizer.
FR-A-2,685,740 describes flexible membranes for hydropneumatic accumulators, which consist of a layer (1) of ethylene/vinyl alcohol (EVOH) copolymer placed between two layers of polyamide (2), the triple-layer itself being coated on each face with a layer of NBR rubber (3). In order to soften the layers of polyamide (2), it is necessary to add EPDM thereto. The layers (2) do not adhere well to the layer (1). It is also necessary to insert an adhesive between the layers (2) and (3).
It is an object of the invention to propose a hydropneumatic accumulator membrane which makes it possible to avoid the drawbacks mentioned above and which is of high stability towards ageing at high temperature.
The subject of the invention is thus a gas-impermeable elastic membrane intended in particular to equip a hydropneumatic accumulator, and comprising at least:
a film formed
of a skin of a gas-impermeable copolymer containing xe2x80x94OH groups,
and of two layers between which is incorporated the skin and which consist of a flexible and elastic material of a copolymer containing polyamide blocks and polyether blocks, a rubber coating optionally being placed on at least one layer of the flexible and elastic material.
The membranes according to the invention are impermeable and thermoformable, have good heat resistance, low rigidification under cold conditions, and are flexible over a wide temperature range without it being necessary to add plasticizers to the gas-impermeable skin, or to the layers of the copolymer containing polyamide blocks and polyether blocks. Furthermore, depending on the types of rubber copolymer used, no adhesive or binder is necessary.
According to another characteristic of the invention, the gas-impermeable copolymer is a copolymer of a monomer of formula CH2xe2x95x90CHxe2x80x94R in which R is H or an alkyl group, with a monomer of formula CH2xe2x95x90CHxe2x80x94(Rxe2x80x2)xOH in which x is equal to 0 or 1 and Rxe2x80x2 is an alkyl group, and more particularly a copolymer of ethylene and of vinyl alcohol.
According to an advantageous form, the membrane of the invention comprises a rubber coating on at least one of its faces and preferably on both faces.
According to other characteristics of the invention:
the rubber is, for example, a nitrile rubber and contains carboxylic, amino or epoxy functional groups,
the melting point of the film formed by the skin and the layers of flexible and elastic material is greater than 160xc2x0 C.,
the thickness of the skin is between 25 and 100 micrometers, and
the thickness of each layer of the flexible material and of the rubber is between 200 and 600 micrometers.
The invention also relates to a process for manufacturing such a membrane. The process consists in carrying out a coextrusion of the skin and the layers in order to obtain a film,. optional drying in order to remove water from the layers, optionally followed by thermoforming in order to give the film the desired shape, and molding with vulcanization of the rubber.
A coextrusion binder can be placed between the skin and the layers of flexible and elastic material.
The drying is preferably carried out for 24 hours at 70xc2x0 C.
The molding with the rubber is carried out at a temperature below the melting point of the film formed by the skin and the layers.
The binding between the rubber and the film can be achieved using an adhesive which is placed beforehand on the outer faces of the film. If the rubber has functional groups, it is not necessary to use an adhesive.
The adhesive comprises a non-polar solvent such as xylene or a ketone.